The present invention relates to the use of gradient refractive index films in electro-optical devices to enhance light or energy propagation. More particularly, the invention relates to the use of gradient refractive index films on light sources, for example OLED light sources, to enhance light extraction and distribution control. It is to be understood, however, that the invention disclosed herein has utility and application in related areas and with other electro-optical devices, including other types of light sources and displays.
Electro-optical devices involve components and systems which operate by modification of the optical properties of a material by an electric field, i.e., by modification of the interaction between the electromagnetic (optical) and the electrical (electronic) states of materials. Such devices include but are not limited to solid state light sources, such as OLEDs and LEDs. Because the devices operate on the principle of generation of energy and the transmittance of that energy through one or more different mediums before the energy is extracted from the device to perform its intended function, for example as a lighting device, there is the potential for energy loss due to a mismatch in the refractive indexes of the various mediums present through which the energy must travel.
Organic light emitting diodes (OLEDs) are known. As is generally understood, an OLED device typically includes one or more organic light emitting layers disposed between electrodes, e.g., a cathode and a light-transmissive anode, formed on a substrate, which may also be light-transmissive. Upon application of an electric current, electrons may be injected into the organic layer from the cathode, and holes may be injected into the organic layer from the anode. The electrons and the holes generally travel through the organic layer until they recombine at a luminescent center, typically an organic molecule or polymer, this recombination process resulting in the emission of a light photon, usually in the visible region of the spectrum. Therefore, as used herein, the term “organic light emitting diode” generally refers to a device including at least electrodes and one or more active layer(s), including an organic material (molecule or polymer), the device exhibiting the characteristic of electroluminescence. Depending on the exact composition of the luminescent center(s) within the light emitting layer, light in one or more color bands may be emitted.
Typically, the layers of an OLED are arranged, as stated above, so that the organic layers are disposed between the cathode and anode layers. As photons of light are generated and emitted, the photons move through the organic layer. Those that move toward the cathode, which generally comprises a non-light transmissive material, for example a metal, may be reflected back into the organic layer. Those photons that move through the organic layer to the light-transmissive anode, and finally to the substrate or other outer surface layer, however, may be emitted from the OLED in the form of light energy. Even though the substrate may be light-transmissive, due to the difference in refractive index of the substrate material with that of the external medium (typically air), some portion of the photons are trapped within the device due to total internal reflection. As the refractive index mismatch becomes larger between two mediums or layers of the device, or between the outermost layer of the device and the external environment, the total internal reflection of the light also increases. Therefore, the trapped photons are not able to escape the device, and thus the OLED operates at less than 100% efficiency with respect to light generated within the device, and subsequently light emitted.
As can be seen from the foregoing, by reducing the refractive index mismatch of the materials through which light energy passes before it is emitted, including the external medium, which is most likely air, light output may be optimized. It would be desirable, therefore, to provide a means for reducing the refractive index mismatch of a light source in order to maximize light output. In addition, it would be desirable to reduce the refractive index mismatch of a light source in order to better optimize light distribution.